Surface-mount filter
BACKGROUND OF THE INVENTION 
     The present invention relates generally to radio frequency (RF) signal filters, and more particularly to a surface-mount filter that couples the printed circuit boards of radio transceivers. 
     Interconnection to prior art ceramic block filters may be accomplished by integral conventional coupling connectors as shown and described in U.S. Pat. No. 4,431,977 or by integral coaxial coupling cables retained by a housing as shown and described in U.S. Pat. No. 4,742,562. However, in both of the foregoing instances coaxial cables are required for interconnection, thereby incurring additional cost. 
     The need for coaxial cables may be eliminated by surface mounting the filter and coupling to the resonators thereof by direct solder connection as shown and described in U.S. Pat. No. 4,673,902. However, direct solder connections to the ceramic block are not reliable since small protrusions i the circuit board may result in open connections. Alternatively, wires may be connected to the resonators as shown and described in U.S. Pat. No. 4,703,291 or to input/output pads as shown and described in U.S. Pat. No. 4,716,391. However, the use of individual wires likewise both increases the manufacturing cost and is unreliable. Therefore, there is need for a surface-mount filter that reliably and inexpensively surface mounts to printed circuit boards of radio transceivers. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved dielectric block filter that reliably and inexpensively surface mounts to printed circuit boards of RF devices. 
     It is another object of the present invention to provide an improved dielectric block filter with shielding bracket, that reliably and inexpensively surface mount to printed circuit boards of RF devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a ceramic block filter, shielding bracket, and printed circuit board, embodying the present invention. 
     FIG. 2 is a cross sectional view of the printed circuit board in FIG. 1 taken along lines 2--2. 
     FIG. 3 is a block diagram of a radio transceiver including shielded ceramic block filters of the present invention for coupling the transmitter and receiver thereof to a common antenna. 
     FIG. 4 is a partial top view of printed circuit board 152 in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is illustrated an exploded perspective view of a filter assembly 100 including ceramic block filter 102, shielding bracket 132, and printed circuit board 152, and embodying the present invention. Filter 102 is a dielectric block filter preferably comprised of a high-dielectric low-loss ceramic, such as those ceramics described in U.S. Pat. Nos. 3,938,064 and 4,431,977. Filter 102 includes electrodes 106 and 107 for interconnection to radio frequency (RF) circuitry of a conventional radio transceiver, such as the transceiver shown and described in Motorola instruction manual number 68P81070E40, entitled &#34;DYNATAC Cellular Mobile Telephone,&#34; published by and available from Motorola C &amp; E Parts, 1313 East Algonquin Road, Schaumburg, Ill. 60196. As illustrated in FIG. 3, shielded ceramic block filters 304 and 305 of the present invention may couple RF receiver 302 and RF transmitter 306, respectively, to a common antenna 308 of a radio transceiver. 
     Filter 102 includes two holes 104 and 105 extending from the top surface to the bottom surface thereof. Although filter 102 is shown having two holes 104 and 105, any number of one or more holes may be used in filters embodying the present invention. The bottom and sides of filter 102 and internal surfaces of holes 104 and 105 are covered with conductive material 160 over substantially their entire surfaces, with the exception of area 110 and a corresponding area on the side opposite to area 110. The top surface of filter 102 may be covered by portions of conductive material 108, 109, 118 and 119 near the periphery thereof, which are electrically connected to the conductive material on the side surfaces of filter 102. Also disposed on the top surface of filter 102 are electrodes 106 and 107 and electrodes 115 and 116. Electrodes 115 and 116 are comprised of conductive material and electrically connected to the conductive material of holes 104 and 105, respectively. Electrodes 106 and 107 are likewise comprised of conductive material. 
     Holes 104 and 105 form transmission line resonators which may be turned by removing conductive material from electrodes 115 and 116, respectively, or from the surrounding conductive material 108, 109, 118, 119 and 160. Holes 104 and 105 have a substantially circular cross section, although other shapes may be used, such as, for example, a rectangular cross section with rounded corners and parallel elongated sides. The electrodes 115 and 116 for holes 104 and 105 may have varying shapes for capacitively coupling to the surrounding conductive material 108, 109, 118, 119 and 160. The amount of capacitive coupling between electrodes 115 and 116 and the surrounding conductive material 108, 109, 118, 119 and 160 ca be varied by decreasing or increasing the gap between electrodes 115 and 116 and the surrounding conductive material 108, 109, 118, 119 and 160, by varying the size of electrodes 115 and 116, or by any other suitable means. Each of the holes 104 and 105 functions essentially as a foreshortened transmission line resonator. In the preferred embodiment, the conductive material covering the surfaces of filter 102 is comprised of silver. 
     Shielding bracket 132 is preferably comprised of a conductive metal (or metallized plastic in other applications) and is shaped to accept filter 102 when inserted therein. The sides of bracket 132 extend at least partially over the corresponding sides of filter 102. Filter 102 may be attached to bracket 132 by means of solder or any other suitable adhesive. Bracket 132 electromagnetically shielding filter 102 from other circuitry. Bracket 132 substantially enclosed the top surface of filter 102 for includes tab 134 and a corresponding tab on the side opposite to tab 134, both of which contact the top side of printed circuit board 152 for positioning bracket 132 and filter 102. Bracket 132 also includes openings 136 and 137 for exposing electrodes 106 and 107 of filter 102. Portions 138 and 139 of bracket 132 extend into opening 136 (and corresponding portions of bracket 132 extend into opening 137) for contacting conductive portions 108 and 109, respectively, of filter 102 for positioning filter 102 in bracket 132. Portions 138 and 139 of bracket 132 (and corresponding portions of bracket 132 extend into opening 137 and position filter 102 relative to tab 134 (and a corresponding tab on the side opposite to tab 134) by an amount approximately equal to the thickness of printed circuit board 152. 
     Printed circuit board 152 includes an opening 162 that is shaped to accept fitler 102 and bracket 132 when inserted therein. Printed circuit board 152 includes transmission lines 156 and 157 which contact and couple electrodes 106 and 107, respectively, of filter 102 to other RF circuitry thereon. According to the present invention, filter 102 and bracket 132 may be surface mounted in hole 162 of printed circuit board 152 and bonded thereto during reflow soldering of printed circuit board 152 and the other components thereon. Transmission lines 156 and 157 are disposed on tabs 158 and 159, respectively, extending into hole 162. Transmission line 156 is shown more clearly in the partial top view of printed circuit board 152 in FIG. 4. Tabs 158 and 159 may vary in width and, in other embodiments, may extend across the entire edge of hole 162. Printed circuit board 152 has a ground plane 154 on its bottom side except for portions near transmission lines 156 and 157. Tab 134 of bracket 132 and a corresponding tab on the side opposite to tab 134 contact and are reflow soldered to grounded portions of circuitry on the top side of printed circuit board 152. 
     According to the present invention, electrodes 106 and 107 of filter 102 are bonded directly to transmission lines 156 and 157 on tabs 158 and 159, respectively, of printed circuit board 152. Since tab 134 of bracket 132 and a corresponding tab on the side opposite to tab 134 support filter 102 in the middle, open circuit connections are avoided between electrode 106 and transmission line 156, and between electrode 107 and transmission line 157. Therefore, by utilizing the present invention, filter 102 and bracket 132 may be reliably and inexpensively surface mounted to printed circuit boards of RF devices. Although shown with two electrodes 106 and 107 and two transmission lines 156 and 157, only one electrode and one transmission line need be used in practicing the present invention. 
     Referring next to FIG. 2, there is illustrated a partial cross sectional view of filter 102, bracket 132, and printed circuit board 152 in FIG. 1 taken through tabs 158 and 159 and transmission lines 156 and 157 along lines 2--2. Ground plane 154 on the bottom of printed circuit board 152 does not extend out under tabs 158 and 159. Tabs 158 and 159 include semicircular plated through holes 164 and 165 disposed at the periphery or edges of tabs 158 and 159. Holes 164 and 165 may also be circular in shape and disposed away from the edges at the ends of tabs 158 and 159, respectively. Holes 164 and 165 extend through transmission lines 156 and 157, respectively, so that the reflow of solder between transmission lines 156 and 157 and electrodes 106 and 107, respectively, of filter 102 may be enhanced and visually inspected. A solder fillet 166 is formed between hole 164 and electrode 106 (and also between hole 165 and electrode 107) when filter 102 has been properly reflow soldered. Therefore, the proper soldering of filter 102 may be determined simply by visually inspecting filter assembly 100 for solder fillet 166. 
     In summary, a unique shielded dielectric block filter reliably and inexpensively surface mounts to printed circuit boards of RF devices, such as radio transceivers. The unique dielectric block filter of the present invention may be advantageously utilized in any application where surface-mount RF filters are desired.